The invention is directed to a method and to an apparatus for controlling an electrographic printing or copying process. The invention is particularly directed to a method and to a system for controlling the toner concentration in a toner/developer mixture.
EP 403 523 B1 discloses an electrographic process wherein a number of process parameters are monitored and regulated, including the toner concentration in a development station. It is provided for this purpose that a toner mark be generated on a photoconductor drum, that the mark be sensed on the photoconductor drum with a reflex light sensor, and the measured value be employed for the control of the toner resupply in the development station. The solid toner mark is thereby generated at the lateral edge of the photoconductor drum, so that it lies only in the region of the margin perforations with respect to the web-shaped, margin-perforated recording medium and thus does not lie in the actual writing region of the recording medium or of the photoconductor drum. A number of disadvantages result therefrom.
First, the toner mark can thus be only designed relatively small, as a result of which the sensing area and, thus, the measuring precision as well are limited, particularly transversely relative to the transport direction of the photoconductor drum or of the recording medium. Second, it is definitely meaningful for regulating various parameters of the electrographic process to also apply and sense a toner mark in the actual writing region of the photoconductor drum. Particularly when employing web-shaped recording media, for example what is referred to as continuous stock, however, spoilage then arises because the recording medium printed in this way is no longer available for being printed with other information. Conditioned by the process, third, edge effects, for example of the electrical field, arise at the edges of a photoconductor, as a result of which the expressiveness of a measurement in this region is diminished. Such edge effects superimpose all the more greatly the narrower a toner mark is.
WO 97/17635 A discloses an electrophotographic printer device that contains compensation elements, particularly rockers, springs and other prestressing elements, that are intended to avoid fluctuations in the position of the recording medium in the region of the transfer printing zone. This is particularly necessary in a duplex mode wherein the web-shaped recording medium is conducted twice through the same transfer printing station. However, nothing noteworthy can be derived from this publication with respect to the regulation of the electrographic process.
DE 198 01 521.6 (applicant""s reference 980101DE) discloses a control method for the toner/developer mixture, what is referred to as a two-component development system, in an electrographic printer or copier device. in this method, a toner mark is sensed at least two places with respect to the transport direction of the photoconductor drum. In order to achieve an adequate sensing precision with this method, it is necessary that the toner mark be adequately broad. It would thereby be especially advantageous if the toner mark could also extend into the actual writing region of the photoconductor drum.
DE 197 49 651.2 (applicant""s reference 971104DE) discloses an electrophotographic printer that can process both recording media with margin perforation as well as recording media without margin perforation. A unit with a friction drive is provided for this purpose and can be pivoted against and away from a photoconductor drum so precisely that the recording medium again comes to lie exactly at the original position with respect to the photoconductor drum after being pivoted away and back in. The components disclosed therein are designed therefor that the pivoting in and away occurs, for example, during the course of a service call. Such a printer, on the other hand, is suited for printing a recording medium without a margin perforation wider than a recording medium with a margin perforation. A recording medium free of margin perforations can then also be printed by the photoconductor drum in the region of the drum wherein the margin perforations are located given a margin-perforated recording medium. This advantage, however, is accompanied by the disadvantage that the toner mark must be transfer-printed onto the unperforated, web-shaped recording medium in its writing region and, thus, spoilage again arises.
U.S. Pat. No. 5,387,965 A and U.S. Pat. No. 4,468,112 A disclose electrographic processes wherein the toner concentration in a development station is identified and compared to a rated value. The rated value is dependent on a second measurement event in which a toner mark is interpreted.
An object of the invention is to specify a method and an electrographic system wherein a monitoring of toner density can occur and wherein an optimally high printing width on the recording medium can be achieved at the same time.
This object is achieved by a method and system of the invention for regulating an electrographic printing or copying process wherein information to be printed are generated as an intermediate toner image on an intermediate image carrier and are later transferred from a transfer printing surface onto a recording medium in a transfer printing zone. A toner mark is generated on an intermediate image carrier such as an electrographic intermediate image carrier. The toner mark on the intermediate image carrier is sensed at least in regions and the toner mark is subsequently in turn removed from the intermediate image carrier. The toner concentration in a development station is regulated with a measured value such as a density value. The transfer of the intermediate toner image from the transfer printing surface onto the recording medium is at least temporarily prevented in the time span between the generation and the removal of the toner mark.
According to a first aspect of the invention, the information to be printed are generated as an intermediate toner image on an intermediate image carrier with a photoconductor drum and, in a transfer printing zone, are transferred later from a transfer printing surface, for example from the surface of the photoconductor drum or from a transfer printing tape, onto a recording medium. For regulating the toner density, a toner mark is generated on the electrographic intermediate image carrier, the toner mark on the intermediate image carrier is sensed at least in regions, and the toner mark is in turn removed from the intermediate image carrier later. In particular, the density is sensed at the toner mark. The toner concentration is then regulated with the measured value, particularly density value, in a development station such that the inking can be kept constant. A transfer of the intermediate toner image from the transfer printing surface onto the recording medium is at least temporarily prevented in the time span between the generation of the toner mark on the intermediate image carrier and the removal of the toner mark from the intermediate image carrier. This impeding transfer of the transfer printing is particularly prevented during a time span wherein the toner mark passes the transfer printing zone. The prevention of the transfer printing can, in particular, occur by separation/lift-off of the recording medium from the transfer printing surface.
In particular, the invention provides two operating states for the operation of an electrographic printer or copier device. In the first operating state, what is referred to as the measurement cycle, a toner mark is generated on the intermediate image carrier, the density of the toner mark is sensed, and the toner mark is then in turn removed from the intermediate image carrier. The sensed toner density value is employed for the regulation of the toner concentration in the development station and, in particular, has influence on a toner concentration rated value and/or on a control threshold. In the second operating state, information to be printed are generated on the intermediate image carrier as a toner image and are transfer-printed onto the recording medium later. In the second operating state, the toner concentration is, in particular, controlled with the toner concentration or regulation threshold determined from the first operating state.
Inventively, the recording medium is at least temporarily separated or lifted off from the transfer printing surface, particularly from the intermediate image carrier, in the first operating state. The invention is therefore suited for generating the toner mark on the intermediate image carrier in a region in which the information to be printed onto the recording medium are also output. The recording medium is, in particular, web-shaped and can, for example, be made of paper.
A photoconductor drum is particularly provided as an intermediate carrier on which the toner mark is generated. The recording medium is separated or lifted off from it for at least one revolution. In a further embodiment of the invention, a photoconductor tape is employed as a first intermediate image carrier and/or a transfer band is employed as a second intermediate image carrier. Information to be printed are thereby generated on the first intermediate image carrier, are subsequently transferred onto the transfer band and are finally transfer-printed from the transfer band onto the recording medium.
In particular, density values of several toner marks can be employed for determining the rated value in the regulation of the toner concentration. The various toner marks can derive from a plurality of measurement cycles that lie far apart in time or can be formed in a continuous measurement cycle, particularly with one or with several revolutions of the photoconductor drum.
A rapidly switchable, high-precision mechanical lift-off device can, in particular, be employed for separating the recording medium from the transfer printing surface. It is thereby particularly provided that the recording medium lies in nearly the same position with respect to the transfer printing station both in the conveying direction as well as transversely relative to the conveying direction before the lift-off and after being pivoted back in. Optionally, the recording medium can be moved or the transfer printing surface or the transfer printing element can be moved for separating the recording medium from the transfer printing surface. Instead of or in addition to the mechanical movement, an electrophotographic quantity in the region of the transfer printing station is modified in a further version for preventing the transfer printing such that a transfer printing is suppressed. For example, the electrical voltage at a transfer printing corotron can be reduced for this purpose.
As a result of the invention, two operating states of the device are created that are employed alternating in time. The operating state wherein the toner mark is generated and interpreted and no transfer printing onto the recording medium occurs is thereby temporally inserted between the other operating state wherein print information are transfer-printed onto the recording medium.
What is especially advantageous about the invention is that the toner mark can be designed very broad, up to the entire width of the intermediate image carrier, for example of a photoconductor drum. As a result thereof, the toner mark can be interpreted at a plurality of locations, particularly according to the method disclosed by DE 198 01 521.6. The content of this application and the WO Publication corresponding with this application is herewith incorporated into the present specification.
The toner mark can also be positioned at any arbitrary location within the printing width of the intermediate image carrier. As a result of these possibilities, not only is the measurement precision enhanced compared to narrow toner marks but the possibility is also created to apply various methods for the interpretation of the toner mark instead of an evaluation of the toner density. For example, an interpretation of the shape of the generated toner mark or the comparison thereof to a rated shape is also possible. This further enables the interpretation methods to be optimized for colored toner. Various interpretation methods can be combined to form an overall interpretation, for example various interpretation methods and/or color-specific properties of the toner mark sensor can be taken into consideration color-specifically with respect to the toner color.
The evaluation method of the toner mark can be designed due to the adaptability of the toner mark in view of its position, size and shape such that independence from the toner color can be achieved. This also particularly applies when an optoelectronic toner mark sensor works in a narrow light wavelength spectrum, for example in only one color, and its light has a direct influence on the reflection and absorption behavior of the toner-pigments. In this case, in particular, the shape recognition of a large-area mark can be advantageously utilized.
In a second aspect of the invention, toner marks in an electrographic process are cyclically generated and sensed only at relatively large page intervals, for example only after a few thousand printed pages. In the intervening printing intervals, the tone concentration is then implemented in some other way, for example based only on other measured quantities. One such other measured quantity can, for example, be the toner concentration, this being measured in the development station with a toner concentration sensor. A rated value, for example for the toner concentration or for a regulation threshold, is then defined from the measured value (toner density, toner mark shape, etc.) determined from the toner mark and the toner concentration is regulated therewith. This regulation of the toner concentration in a development station to be applied between the toner mark measurements can occur in a known way.
Corresponding to the second aspect of the invention, the toner replenishment is regulated in some other way over relatively large printing intervals (between the tone mark evaluation cycles). Instead of or in addition to the toner concentration control that has just been described, the toner consumption can also be identified on the basis of the printed information, particularly as a result of the total number of printed picture elements (pixels). This determination can also occur dependent on the contrast ratio set at the printer device, on a specific transfer printing efficiency, on the age of the toner/developer mixture, on the toner type and/or on other influencing variables. The delivery of toner into the development station from a toner reservoir can then be controlled with quantities defined in this way.
The second aspect of the invention achieves the object of being able to implement a largely independent regulation of the toner inking in an electrographic process in the intervals between the formation and measurement of the toner mark.